Dominant feminizing mutations implicate protein-protein interactions as the main mode of regulation of the nematode sex-determining gene tra-1

Sex Determination in Nematode Germ Cells

BACKGROUND

Animal germ cells differentiate as sperm or as oocytes. These sexual fates are controlled by complex regulatory pathways to ensure that the proper gametes are made at the appropriate times.

SUMMARY

Nematodes like Caenorhabditis elegans and its close relatives are ideal models for studying how this regulation works, because the XX animals are self-fertile hermaphrodites that produce both sperm and oocytes. In these worms, germ cells use the same signal transduction pathway that functions in somatic cells. This pathway determines the activity of the transcription factor TRA-1, a Gli protein that can repress male genes. However, the pathway is extensively modified in germ cells, largely by the action of translational regulators like the PUF proteins. Many of these modifications play critical roles in allowing the XX hermaphrodites to make sperm in an otherwise female body. Finally, TRA-1 cooperates with chromatin regulators in the germ line to control the activity of fog-1 and fog-3, which are essential for spermatogenesis. FOG-1 and FOG-3 work together to determine germ cell fates by blocking the translation of oogenic transcripts. Key Messages: Although there is great diversity in how germ cell fates are controlled in other animals, many of the key nematode genes are conserved, and the critical role of translational regulators may be universal.

Sexual modulation of sex-shared neurons and circuits in Caenorhabditis elegans

Studies using the nematode C. elegans have provided unique insights into the development and function of sex differences in the nervous system. Enabled by the relative simplicity of this species, comprehensive studies have solved the complete cellular neuroanatomy of both sexes as well as the complete neural connectomes of the entire adult hermaphrodite and the adult male tail. This work, together with detailed behavioral studies, has revealed three aspects of sex differences in the nervous system: sex-specific neurons and circuits; circuits with sexually dimorphic synaptic connectivity; and sex differences in the physiology and functions of shared neurons and circuits. At all of these levels, biological sex influences neural development and function through the activity of a well-defined genetic hierarchy that acts throughout the body to translate chromosomal sex into the state of a master autosomal regulator of sexual differentiation, the transcription factor TRA-1A. This Review focuses on the role of genetic sex in implementing sex differences in shared neurons and circuits, with an emphasis on linking the sexual modulation of specific neural properties to the specification and optimization of sexually divergent and dimorphic behaviors. An important and unexpected finding from these studies is that chemosensory neurons are a primary focus of sexual modulation, with genetic sex adaptively shaping chemosensory repertoire to guide behavioral choice. Importantly, hormone-independent functions of genetic sex are the principal drivers of all of these sex differences, making nematodes an excellent model for understanding similar but poorly understood mechanisms that likely act throughout the animal kingdom. © 2016 Wiley Periodicals, Inc.

p53 modulates the activity of the GLI1 oncogene through interactions with the shared coactivator TAF9

The GLI1 oncogene and p53 tumor suppressor gene function in an inhibitory loop that controls stem cell and tumor cell numbers. Since GLI1 and p53 both interact with the coactivator TATA Binding Protein Associated Factor 9 (TAF9), we hypothesized that competition between these transcription factors for TAF9 in cancer cells may contribute to the inhibitory loop and directly affect GLI1 function and cellular phenotype. We showed that TAF9 interacts with the oncogenic GLI family members GLI1 and GLI2 but not GLI3 in cell-free pull-down assays and with GLI1 in rhabdomyosarcoma and osteosarcoma cell lines. Removal of the TAF9-binding acidic alpha helical transactivation domain of GLI1 produced a significant reduction in the ability of GLI1 to transform cells. We then introduced a point mutation into GLI1 (L1052I) that eliminates TAF9 binding and a point mutation into GLI3 (I1510L) that establishes binding. Wild-type and mutant GLI proteins that bind TAF9 showed enhanced transactivating and cell transforming activity compared with those that did not. Therefore, GLI-TAF9 binding appears important for oncogenic activity. We then determined whether wild-type p53 down-regulates GLI function by sequestering TAF9. We showed that p53 binds TAF9 with greater affinity than does GLI1 and that co-expression of p53 with GLI1 or GLI2 down-regulated GLI-induced transactivation, which could be abrogated using mutant forms of GLI1 or p53. This suggests that p53 sequesters TAF9 from GLI1, which may contribute to inhibition of GLI1 activity by p53 and potentially impact therapeutic success of agents targeting GLI-TAF9 interactions in cancer.

Developmental alterations of the C. elegans male anal depressor morphology and function require sex-specific cell autonomous and cell non-autonomous interactions

We studied the Caenorhabditis elegans anal depressor development in larval males and hermaphrodites to address how a differentiated cell sex-specifically changes its morphology prior to adulthood. In both sexes, the larval anal depressor muscle is used for defecation behavior. However in the adult males, the muscle's sarcomere is reorganized to facilitate copulation. To address when the changes occur in the anal depressor, we used YFP:actin to monitor, and mutant analysis, laser-ablation and transgenic feminization to perturb the cell's morphological dynamics. In L1 and L2 stage larva, the muscle of both sexes has similar sarcomere morphology, but the hermaphrodite sex-determination system promotes more growth. The male anal depressor begins to change in the L3 stage, first by retracting its muscle arm from the neurons of the defecation circuit. Then the muscle's ventral region develops a slit that demarcates an anterior and posterior domain. This demarcation is not dependent on the anal depressor's intrinsic genetic sex, but is influenced by extrinsic interactions with the developing male sex muscles. However, subsequent changes are dependent on the cell's sex. In the L4 stage, the anterior domain first disassembles the dorsal-ventral sarcomere region and develops filopodia that elongates anteriorly towards the spicule muscles. Later, the posterior domain dissembles the remnants of its sarcomere, but still retains a vestigial attachment to the ventral body wall. Finally, the anterior domain attaches to the sex muscles, and then reassembles an anterior-posteriorly oriented sarcomere. Our work identifies key steps in the dimorphic re-sculpting of the anal depressor that are regulated by genetic sex and by cell-cell signaling.

A sexually conditioned switch of chemosensory behavior in C. elegans

In sexually reproducing animals, mating is essential for transmitting genetic information to the next generation and therefore animals have evolved mechanisms for optimizing the chance of successful mate location. In the soil nematode C. elegans, males approach hermaphrodites via the ascaroside pheromones, recognize hermaphrodites when their tails contact the hermaphrodites' body, and eventually mate with them. These processes are mediated by sensory signals specialized for sexual communication, but other mechanisms may also be used to optimize mate location. Here we describe associative learning whereby males use sodium chloride as a cue for hermaphrodite location. Both males and hermaphrodites normally avoid sodium chloride after associative conditioning with salt and starvation. However, we found that males become attracted to sodium chloride after conditioning with salt and starvation if hermaphrodites are present during conditioning. For this conditioning, which we call sexual conditioning, hermaphrodites are detected by males through pheromonal signaling and additional cue(s). Sex transformation experiments suggest that neuronal sex of males is essential for sexual conditioning. Altogether, these results suggest that C. elegans males integrate environmental, internal and social signals to determine the optimal strategy for mate location.

A CUL-2 ubiquitin ligase containing three FEM proteins degrades TRA-1 to regulate C. elegans sex determination

In Caenorhabditis elegans, the Gli-family transcription factor TRA-1 is the terminal effector of the sex-determination pathway. TRA-1 activity inhibits male development and allows female fates. Genetic studies have indicated that TRA-1 is negatively regulated by the fem-1, fem-2, and fem-3 genes. However, the mechanism of this regulation has not been understood. Here, we present data that TRA-1 is regulated by degradation mediated by a CUL-2-based ubiquitin ligase complex that contains FEM-1 as the substrate-recognition subunit, and FEM-2 and FEM-3 as cofactors. CUL-2 physically associates with both FEM-1 and TRA-1 in vivo, and cul-2 mutant males share feminization phenotypes with fem mutants. CUL-2 and the FEM proteins negatively regulate TRA-1 protein levels in C. elegans. When expressed in human cells, the FEM proteins interact with human CUL2 and induce the proteasome-dependent degradation of TRA-1. This work demonstrates that the terminal step in C. elegans sex determination is controlled by ubiquitin-mediated proteolysis.

A complex solution to a sexual dilemma

The C. elegans male sex-determining protein, FEM-1, has been identified as a substrate recognition subunit of a Cullin-2 ubiquitin ligase complex. This complex controls the level of TRA-1A, a Ci/Gli homolog and master regulator of sex determination, by ubiquitin-mediated proteolysis.

Genetic control of sex differences in C. elegans neurobiology and behavior

As a well-characterized, genetically tractable animal, the nematode Caenorhabditis elegans is an ideal model to explore the connections between genes and the sexual regulation of the nervous system and behavior. The two sexes of C. elegans, males and hermaphrodites, have precisely defined differences in neuroanatomy: superimposed onto a "core" nervous system of exactly 294 neurons, hermaphrodites and males have 8 and 89 sex-specific neurons, respectively. These sex-specific neurons are essential for cognate sex-specific behaviors, including hermaphrodite egg-laying and male mating. In addition, regulated sex differences in the core nervous system itself may provide additional, though poorly understood, controls on behavior. These differences in the nervous system and behavior, like all known sex differences in the C. elegans soma, are controlled by the master regulator of C. elegans sex determination, tra-1. Downstream of tra-1 lie specific effectors of sex determination, including genes controlling sex-specific cell death and a family of regulators, the DM-domain genes, related to Drosophila doublesex and the vertebrate DMRT genes. There is no central (i.e., gonadal) regulator of sexual phenotype in the C. elegans nervous system; instead, tra-1 acts cell-autonomously in nearly all sexually dimorphic somatic cells. However, recent results suggest that the status of the gonad can be communicated to the nervous system to modulate sex-specific behaviors. Continuing research into the genetic control of neural sex differences in C. elegans is likely to yield insight into conserved mechanisms of cell-autonomous cross talk between cell fate patterning and sexual differentiation pathways.

The C. elegans Sex-Determining GLI Protein TRA-1A Is Regulated by Sex-Specific Proteolysis

TRA-1A is the sole representative in Caenorhabditis elegans of the Gli transcription factor family. Its activity is required to specify all somatic female cell fates in XX hermaphrodites. We have found that TRA-1 protein levels are much higher in hermaphrodites than in males, and that the difference is attributable to the predominance in hermaphrodites of C-terminally truncated isoforms that are nearly undetectable in males. Our results support a model in which TRA-1A is negatively regulated by male-specific proteolysis that depends upon specific TRA-1A protein sequences and upon the activity of the fem genes. C-terminally truncated TRA-1 isoforms are stable and can inappropriately feminize XO males, suggesting that they escape this negative regulation. Thus, although C. elegans appears to lack a Hedgehog-signaling pathway, our results indicate that proteolytic processing and degradation of Gli family transcription factors, commonly seen during Hedgehog signaling in other organisms, also control C. elegans sex determination.

Conservation of the global sex determination gene tra-1 in distantly related nematodes

Sex determination has long intrigued evolutionists, geneticists, and developmental biologists in a similar way. Substantial evidence indicates that sex determination evolves rapidly and, therefore, can be used to study how molecular patterning processes evolve. In Caenorhabditis elegans, sex determination relies on a signaling pathway that involves a cascade of negatively acting factors, finally triggering the GLI-family zinc-finger transcription factor TRA-1. We have started to investigate sex determination in the nematode satellite species Pristionchus pacificus that is separated from C. elegans for 200-300 million years. In P. pacificus, animals with two X chromosomes develop as hermaphrodites, whereas XO animals develop as males. We used an unbiased forward genetic approach and isolated several mutants with a hermaphrodite to male transformation of the XX karyotype. We identified one complementation group as representing the P. pacificus ortholog of tra-1, providing the first evidence for the conservation of a global sex determination gene over a time period of at least 200 million years. A Ppa-tra-1 morpholino phenocopies Ppa-tra-1 mutants and establishes the morpholino technology as a reverse genetic approach in P. pacificus.

The evolution of signalling pathways in animal development

Despite the bewildering number of cell types and patterns found in the animal kingdom, only a few signalling pathways are required to generate them. Most cell-cell interactions during embryonic development involve the Hedgehog, Wnt, transforming growth factor-beta, receptor tyrosine kinase, Notch, JAK/STAT and nuclear hormone pathways. Looking at how these pathways evolved might provide insights into how a few signalling pathways can generate so much cellular and morphological diversity during the development of individual organisms and the evolution of animal body plans.

Turning clustering loops: sex determination in Caenorhabditis elegans

The nematode Caenorhabditis elegans has two sexes: males and hermaphrodites. Hermaphrodites are essentially female animals that produce sperm and oocytes. In the past few years tremendous progress has been made towards understanding how sexual identity is controlled in the worm. These analyses have revealed that the regulatory pathway controlling sexual development is far from linear and that it contains a number of loops and branches that play crucial roles in regulating sexual development. This review summarizes our current understanding of the mechanisms that regulate sexual cell fate in C. elegans.

RNA-Regulated TRA-1 nuclear export controls sexual fate

TRA-1, a member of the GLI family of transcription factors, is required for C. elegans female development. We find that TRA-1 has a sex-specific distribution consistent with its role in female development: nuclear TRA-1 is higher in hermaphrodite intestines and in specific germline regions than in males. TRA-1 patterns rely on nuclear export since treatment with leptomycin B, a CRM1-dependent export inhibitor, increases nuclearTRA-1 in males. TRA-1 export requires TRA-1 binding to the tra-2 3' untranslated region (3' UTR), as disruption of binding increases nuclear TRA-1 and female development. Our data are consistent with coexport of a TRA-1/tra-2 mRNA complex reducing TRA-1 nuclear activity, and identify an interesting RNA-based mechanism for controlling transcriptional activity and cell fate determination.

The TRA-1 transcription factor binds TRA-2 to regulate sexual fates in Caenorhabditis elegans

The tra-1 and tra-2 sex-determining genes promote female fates in Caenorhabditis elegans. Classical genetic studies placed tra-1 as the terminal regulator of the pathway with tra-2 acting upstream as a regulator of regulators of tra-1. Here we report the surprising result that the TRA-1 transcription factor binds the intracellular domain of the TRA-2 membrane protein. This binding is dependent on the MX regulatory domain, a region of the TRA-2 intracellular domain shown previously to be critical for the onset of hermaphrodite spermatogenesis. The functional importance of the TRA-1-TRA-2 physical interaction is supported by genetic interactions between tra-1(0) and tra-2(mx) mutations: a reduction of tra-1 gene dose from two copies to one copy enhances the tra-2(mx) feminization phenotype, but has no apparent somatic effect. In Caenorhabditis briggsae, we also find an MX-dependent interaction between Cb-TRA-1 and Cb-TRA-2, but intriguingly, no cross-species interactions are seen. The conservation of the TRA-1- TRA-2 interaction underscores its importance in sex determination.

GLI-2 modulates retroviral gene expression

GLI proteins are involved in the development of mice, humans, zebrafish, Caenorhabditis elegans, Xenopus, and Drosophila. While these zinc finger-containing proteins bind to TG-rich promoter elements and are known to regulate gene expression in C. elegans and Drosophila, mechanistic understanding of how regulation is mediated through naturally occurring transcriptional promoters is lacking. One isoform of human GLI-2 appears to be identical to a factor previously called Tax helper protein (THP), thus named due to its ability to interact with a TG-rich element in the human T-lymphotropic virus type 1 (HTLV-1) enhancer thought to mediate transcriptional stimulation by the Tax protein of HTLV-1. We now demonstrate that, working through its TG-rich binding site and adjacent elements, GLI-2/THP actually suppresses gene expression driven by the HTLV-1 promoter. GLI-2/THP has no effect on the HTLV-2 promoter, activates expression from the promoters of human immunodeficiency virus types 1 and (HIV-1 and -2), and stimulates HIV-1 replication. Both effective suppression and activation of gene expression and viral replication require the first of the five zinc fingers, which is not necessary for DNA binding, to be intact. Thus, not only can GLI-2/THP either activate or suppress gene expression, depending on the promoter, but the same domain (first zinc finger) mediates both effects. These findings suggest a role for GLI-2 in retroviral gene regulation and shed further light on the mechanisms by which GLI proteins regulate naturally occurring promoters.

Placental cell fates are regulated in vivo by HIF-mediated hypoxia responses

Placental development is profoundly influenced by oxygen (O(2)) tension. Human cytotrophoblasts proliferate in vitro under low O(2) conditions but differentiate at higher O(2) levels, mimicking the developmental transition they undergo as they invade the placental bed to establish the maternal-fetal circulation in vivo. Hypoxia-inducible factor-1 (HIF-1), consisting of HIF-1alpha and ARNT subunits, activates many genes involved in the cellular and organismal response to O(2) deprivation. Analysis of Arnt(-/-) placentas reveals an aberrant cellular architecture due to altered cell fate determination of Arnt(-/-) trophoblasts. Specifically, Arnt(-/-) placentas show greatly reduced labyrinthine and spongiotrophoblast layers, and increased numbers of giant cells. We further show that hypoxia promotes the in vitro differentiation of trophoblast stem cells into spongiotrophoblasts as opposed to giant cells. Our results clearly establish that O(2) levels regulate cell fate determination in vivo and that HIF is essential for mammalian placentation. The unique placental phenotype of Arnt(-/-) animals also provides an important tool for studying the disease of preeclampsia. Interestingly, aggregation of Arnt(-/-) embryonic stem (ES) cells with tetraploid wild-type embryos rescues their placental defects; however, these embryos still die from yolk sac vascular and cardiac defects.

The C. elegans cell death specification gene ces-1 encodes a snail family zinc finger protein

The ces-1 and ces-2 genes of C. elegans control the programmed deaths of specific neurons. Genetic evidence suggests that ces-2 functions to kill these neurons by negatively regulating the protective activity of ces-1, ces-2 encodes a protein closely related to the vertebrate PAR family of bZIP transcription factors, and a ces-2/ces-1-like pathway may play a role in regulating programmed cell death in mammalian lymphocytes. Here we show that ces-1 encodes a Snail family zinc finger protein, most similar in sequence to the Drosophila neuronal differentiation protein Scratch. We define an element important for ces-1 regulation and provide evidence that CES-2 can bind to a site within this element and thus may directly repress ces-1 transcription. Our results suggest that a transcriptional cascade controls the deaths of specific cells in C. elegans.

The TRA-1A sex determination protein of C. elegans regulates sexually dimorphic cell deaths by repressing the egl-1 cell death activator gene

The hermaphrodite-specific neurons (HSNs) of the nematode Caenorhabditis elegans are generated embryonically in both hermaphrodites and males but undergo programmed cell death in males. The gene egl-1 encodes a BH3-containing cell death activator that is required for programmed cell death in C. elegans. Gain-of-function (gf) mutations in egl-1 cause the inappropriate programmed cell death of the HSNs in hermaphrodites. These mutations lie 5.6 kb downstream of the egl-1 transcription unit and disrupt the binding of the TRA-1A zinc finger protein, the terminal global regulator of somatic sexual fate. This disruption results in the activation of the egl-1 gene in the HSNs not only in males but also in hermaphrodites. Our findings suggest that in hermaphrodites TRA-1A represses egl-1 transcription in the HSNs to prevent these neurons from undergoing programmed cell death.

Sex and the single worm: sex determination in the nematode C. elegans

The study of sex determination in model organisms has been especially fruitful in increasing our understanding of developmental biology, gene regulation and evolutionary mechanisms. The free living nematode, Caenorhabditis elegans, can develop as one of two sexes; male or self-fertilizing hermaphrodite. Here we discuss the progress toward a genetic and molecular understanding of that decision. Numerous genetic loci have been identified that affect sexual fate, and epistasis analysis of these genes has led to a model of a regulatory hierarchy with stepwise negative interactions. It is becoming evident that many of the genes have numerous levels of regulation. We also discuss the apparent rapid rate of evolution that many of the sex determination proteins have undergone. Protein sequences of homologues from closely related species are more divergent than homologues of proteins involved in other developmental processes. Rapid evolution of sex determination genes may be a common theme throughout the animal kingdom.

GLI activates transcription through a herpes simplex viral protein 16-like activation domain

Three proteins have been identified in mammals, GLI, GLI2, and GLI3, which share a highly conserved zinc finger domain with Drosophila Cubitus interruptus and are believed to function as transcription factors in the vertebrate Sonic hedgehog-Patched signaling pathway. To understand the role GLI plays in the Sonic hedgehog-Patched pathway and mechanisms of GLI-induced transcriptional regulation, we have characterized its transcriptional regulatory properties and contributions of specific domains to transcriptional regulation. We have demonstrated that GLI activates expression of reporter constructs in HeLa cells in a concentration-dependent manner through the GLI consensus binding motif and that a GAL4 binding domain-GLI fusion protein activates reporter expression through the GAL4 DNA binding site. GLI-induced transcriptional activation requires the carboxyl-terminal amino acids 1020-1091, which includes an 18-amino acid region highly similar to the alpha-helical herpes simplex viral protein 16 activation domain, including the consensus recognition element for the human TFIID TATA box-binding protein-associated factor TAFII31 and conservation of all three amino acid residues believed to contact directly chemically complementary residues in TAFII31. The presence of this region in the GLI activation domain provides a mechanism for GLI-induced transcriptional regulation.

Conservation of the C.elegans tra-2 3'UTR translational control

The Caenorhabditis elegans sex-determination gene, tra-2, is translationally regulated by two 28 nt elements (DREs) located in the 3'UTR that bind a factor called DRF. This regulation requires the laf-1 gene activity. We demonstrate that the nematode Caenorhabditis briggsae tra-2 gene and the human oncogene GLI are translationally regulated by elements that are functionally equivalent to DREs. Here, we rename the DREs to TGEs (tra-2 and GLI elements). Similarly to the C.elegans tra-2 TGEs, the C.briggsae tra-2 and GLI TGEs repress translation of a reporter transgene in a laf-1 dependent manner. Furthermore, they regulate poly(A) tail length and bind DRF. We also find that the C.elegans TGEs control translation and poly(A) tail length in C.briggsae and rodent cells. Moreover, these same organisms contain a factor that specifically associates with the C.elegans TGEs. These findings are consistent with the TGE control being present in C.briggsae and rodent cells. Three lines of evidence indicate that C.briggsae tra-2 and GLI are translationally controlled in vivo by TGEs. First, like C.elegans tra-2 TGEs, the C.briggsae tra-2 and GLI TGEs control translation and poly(A) tail lengths in C.briggsae and rodent cells, respectively. Second, the same factor in C.briggsae and mammalian cells that binds to the C.elegans tra-2 TGEs binds the C.briggsae tra-2 and GLI TGEs. Third, deletion of the GLI TGE increases GLI's ability to transform cells. These findings suggest that TGE control is conserved and regulates the expression of other mRNAs.

A genetic pathway for regulation of tra-2 translation

In Caenorhabditis elegans, the tra-2 sex-determining gene is regulated at the translational level by two 28 nt direct repeat elements (DREs) located in its 3′ untranslated region (3′UTR). DRF is a factor that binds the DREs and may be a trans-acting translational regulator of tra-2. Here we identify two genes that are required for the normal pattern of translational control. A newly identified gene, called laf-1, is required for translational repression by the tra-2 3′UTR. In addition, the sex-determining gene, tra-3, appears to promote female development by freeing tra-2 from laf-1 repression. Finally, we show that DRF activity correlates with translational repression of tra-2 during development and that tra-3 regulates DRF activity. We suggest that tra-3 may promote female development by releasing tra-2 from translation repression by laf-1 and that translational control is important for proper sex determination--both in the early embryo and during postembryonic development.

Vive la différence: males vs females in flies vs worms

For 600 million years, the two best-understood metazoan species, the nematode Caenorhabditis elegans and fruit fly Drosophila melanogaster, have developed independent strategies for solving a biological problem faced by essentially all metazoans: how to generate two sexes in the proper proportions. The genetic program for sexual dimorphism has been a major focus of research in these two organisms almost from the moment they were chosen for study, and it may now be the best-understood general aspect of their development. In this review, we compare and contrast the strategies used for sex determination (including dosage compensation) between "the fly" and "the worm" and the way this understanding has come about. Although no overlap has been found among the molecules used by flies and worms to achieve sex determination, striking similarities have been found in the genetic strategies used by these two species to differentiate their sexes.

Caenorhabditis elegans sex-determining protein FEM-2 is a protein phosphatase that promotes male development and interacts directly with FEM-3

Male sexual development in the nematode Caenorhabditis elegans requires the genes fem-1, fem-2, and fem-3. The current model of sex determination portrays the FEM proteins as components of a novel signal transduction pathway, but the mechanisms involved in signaling through the pathway are not understood. We report the isolation of fem-2 cDNAs in a yeast two-hybrid screen for clones encoding proteins that interact with FEM-3. Association of FEM-3 and FEM-2 in two independent in vitro binding assays substantiates the interaction detected in the two-hybrid system. FEM-2 is related in sequence to protein serine/threonine phosphatases of Type 2C (PP2C). We demonstrate that FEM-2 exhibits magnesium-dependent casein phosphatase activity, typical of PP2C, in vitro. Point mutations that abolish the casein phosphatase activity of FEM-2 without affecting its FEM-3-binding activity reduce severely its ability to rescue male development in fem-2 mutant nematodes. These results suggest that protein phosphorylation regulates sex determination in C. elegans.